In the solution polymerization of cycloolefins, the product discharged from the reactor is a honey-like cement that consists principally of a nonpolar carrier solvent, such as toluene or cyclohexane, in which the polymer is dissolved. The polymer content is normally on the order of about 15% by weight. The polymer can be any of the family of polymers that are made by homopolymerization or copolymerization of one or more of cycloolefins that contain the norbornene group.
After the honey-like cement is prepared, it is necessary to separate the polymer from its carrier solvent. In the past, steam stripping has been used exclusively in plant operations to extract the polymer from the carrier solvent. In steam stripping, the cement is injected into a jet of steam that is directed into a vessel containing hot water. As contact is made between the cement and a jet of steam, the carrier solvent is flashed off as vapor, depositing the polymer in particle form in the hot water.
Steam stripping has a number of serious disadvantages. It produces a product of relatively large, coarse and a variable particle size. The product contains a substantial amount of occluded water that makes it extremely difficult to dry. It produces a product that retains significant quantities of residual monomers and other residues that include high boiling reaction products and catalyst residue, all of which adversely affect the quality of the final product. Steam stripping has the inherent carry-over problem of polymer fines with the solvent vapor and steam that is continuously discharged. This carry-over results in severe plugging in the solvent recovery system. Finally, steam stripping requires large volumes of steam, far in excess of that required to vaporize the solvent in order to produce a particulate product. Steam stripping, therefore, is an inefficient, expensive, and energy-wasteful operation.
It has been a common practice to reduce impurities in the polymers of cycloolefins by water-washing the polymers. In this way, all or most of the water-soluble impurities are removed with the water. Water-washing has not been practical since large volumes of contaminated water was produced that had to be disposed without creating an environmental problem.
More recently, an alternate approach was discovered for isolating polymers of cycloolefins from the carrier solvent. Pursuant to this approach, the cement is mixed with a nonsolvent in the volume ratio of about 3 to 1, nonsolvent to cement, in a high shear mixer whereby the polymer precipitates out. A nonsolvent is a liquid that is miscible with the nonpolar solvent that is used in the polymerization reaction but is a nonsolvent for the polymer. Although on some occasions this recovery procedure produced granular, easy-to-dry product having bulk density of about 9 lb/ft.sup.3, these results could not be reliably reproduced. What was obtained normally was a clump-like product of fine, irregular fluffy microfibers that pack cotton-like when filtered and are difficult to dry, the dry product having bulk density below 5 lb/ft.sup.3. When polymer cement is precipitated or coagulated in a nonsolvent medium, the high polymers appear to precipitate from and the oligomers remain solubilized in the nonsolvent medium. Since a substantial portion of the impurities are soluble in the nonsolvent, this recovery process succeeded in removing the bulk of the impurities from the polymer, however, this approach was not entirely successful since large volumes of contaminated liquid was produced composed primarily of nonsolvent, nonpolar or reaction solvent, and impurities that included residual shortstop for the polymerization reaction, adducts of the shortstop with catalyst residues, residual catalyst components, oligomers, etc. Solvent recovery of the large volume of nonsolvent--solvent liquid is difficult and expensive.
As already described, mixing large volume of nonsolvent with the polymer cement caused precipitation of polymer in a fluffy, clump-like mass. Since it was anticipated that such a fluffy, clumpy product would cause processing and handling difficulties, an idea was conceived that addition of a small amount of a nonsolvent might facilitate nucleation of the polymer and subsequent precipitation of the polymer in a granular or other form that is preferred to the fluffy, clump-like mass. An experiment was carried out by mixing a honey-like polymer cement with 30% by volume, based on the volume of the cement, of ethanol. The cement was a 15% total solids solution of polymeric cycloolefins in cyclohexane. Unexpectedly, a two-phase system formed, the upper phase being a thin hydrocarbon liquid and the lower phase being a viscous, gel-like cement. Also unexpected was the fact that the lower phase had solids concentration of about 35% and that about 70% of the cyclohexane solvent was extracted with only 5% of the nonsolvent that is required for precipitation of the polymer from polymer cement.